This invention relates to the production of furfural from lignosulphonate waste liquor.
Depending on the particular wood processed, the sulphite pulping process results in a waste liquor which may contain from 0.9 to 5.6 percent of pentose which makes it a candidate for the production of furfural.
Conventionally, the waste liquor is thickened to a solids content of 50 percent or totally dried, the liquor or solid being sold as lignosulphonate. Pentose is an undesirable constituent of this liquor.
The waste liquor from the process is saturated with calcium sulphate, so that the high temperature required for furfural production (preferably above 200 degrees Centigrade) results in the rapid and severe deposition of calcium sulphate on the heated surfaces, which renders extended continuous operation impossible.
If heat exchangers are avoided by substitution of direct steam injection, it is not possible thermodynamically to keep the liquor in a state of boiling during its residence time because the substances in solution elevate the boiling point Thus, at any pressure, the boiling point of the liquor is higher than the condensation point of steam. Consequently, the furfural produced from the pentose remains temporarily dissolved in the liquid phase where, under the catalytic effect of the liquor""s innate acidity, the furfural can react with pentose, or the lignosulphonate, and with itself, thus incurring great losses and consequent poor yields.
It is an object of the present invention to provide a process for the efficient production of furfural from lignosulphonate liquors as well as apparatus for carrying out the process.
According to the invention a process for the production of furfural from lignosulphonate waste liquor containing pentose is characterised by maintaining the liquor at boiling point by controlled decompression for a time sufficient for the pentose to be converted to furfural and to be removed in the vapor substantially without reacting with pentose, lignosulphonate or itself.
The reactor may be a batch or continuous reactor.
In a preferred form batch process, a lignosulphonate liquor is heated by steam, in a reactor which is continuously depressurised to pressures which are sufficient to maintain boiling of the liquor, the furfural formed migrating to the vapor phase and is discharged with the condensate and recovered.
In a continuous process according to the invention, the liquor is boiled in a continuous reactor by means of an auxiliary heat source, the liquor being discharged while the furfural formed is substantially instantaneously and completely transferred into the gas phase leaving the reactor and from which it is separated.
In a form of the invention the liquor is heated in the reactor by means of an auxiliary beat source, the liquor being discharged while the furfural formed is substantially instantaneously and completely transferred into the gas phase from which it separated.
The auxiliary heat source is preferably heated air under pressure and this may be introduced at a low level in the reactor. The air percolates through the liquor in the reactor, and by giving up its heat it maintains the liquor in a state of boiling before leaving the reactor.
The liquor is preferably introduced into the reactor at a temperature of between 180 and 280 degrees Centigrade.
A control circuit may be provided to maintain the pressure in the head of the reactor at a value slightly below the pressure of the entering liquor. In this way the resulting liberating steam causes the liquor to undergo a minor depression to a lower temperature which forces the furfural to the vapor phase, while the heated air maintains a state of boiling of the liquor throughout the entire reactor. Thus, the furfural produced from the pentose is immediately and completely vapourised as it is formed; and joins the air and some vapourised water to form a gaseous mixture with minimum yield loss of furfural, because reactions between furfural and pentose on the one hand, and with the lignosulphonate on the other hand cannot take place because the pentose and lignosulphonate remain in solution. The reaction of furfural with itself is prevented by the absence of hydrogen ions in the vapour phase.
The residence time in the reactor is chosen for there to be complete conversion of the pentose. No addition of acid is required because of the innate acidity of the liquor which effects a sufficiently strong catalysis.
No heat exchangers are required, thereby avoiding the problems associated with calcium sulphate as discussed above.
Instead of using heated air as the auxiliary heat source, other suitable heated gases or gas mixtures (such as hot combustion gas) or hot mercury may be used, all of which are readily separated from the liquor at the end of the reaction.
It will be appreciated that the heat required by the auxiliary heat agent is relatively small, as in essence all that is required of it is to vapourise the furfural produced. The low heat of vapourisation of furfural, especially at high temperatures, ensures a low auxiliary heat requirement.
The yield of furfural increases with the increasing temperature in the reactor as the losses due to reactions with pentose and lignosulphonate as well as bisulphites are suppressed at higher temperatures on account of the entropy effect on all aggregation reactions.
It will be appreciated that it is not necessary to add any acid to the liquor for catalysis, as the innate acidity of the liquor effects a sufficiently strong catalysis.